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152.2.1 Well-Mixed Greenhouse Gases
AR4 (Forster et al., 2007; IPCC, 2007) concluded that increasing atmospheric burdens of well-mixed GHGs resulted in a 9% increase in their RF from 1998 to 2005. Since 2005, the atmospheric abundances of many well-mixed GHG increased further, but the burdens of some ozone-depleting substances (ODS) whose production and use were controlled by the Montreal Protocol on Substances that Deplete the Ozone Layer (1987; hereinafter, ‘Montreal Protocol’) decreased. Based on updated in situ observations, this assessment concludes that these trends resulted in a 7.5% increase in RF from GHGs from 2005 to 2011, with carbon dioxide (CO₂) contributing 80%. Of note is an increase in the average growth rate of atmospheric methane (CH4) from ~0.5 ppb/yr during 1999–2006 to ~6 ppb/yr from 2007 through 2011. Current observation networks are sufficient to quantify global annual mean burdens used to calculate RF and to constrain global emission rates (with knowledge of loss rates), but they are not sufficient for accurately estimating regional scale emissions and how they are changing with time. The globally, annually averaged well-mixed GHG mole fractions reported here are used in Chapter 8 to calculate RF. A direct, inseparable connection exists between observed changes in atmospheric composition and well-mixed GHG emissions and losses (discussed in Chapter 6 for CO₂, CH4, and N₂O). A global GHG budget consists of the total atmospheric burden, total global rate of production or emission (i.e., sources), and the total global rate of destruction or removal (i.e., sinks). Precise, accurate systematic observations from independent globally distributed measurement networks are used to estimate global annual mean well-mixed GHG mole fractions at the Earth’s surface, and these allow estimates of global burdens. Emissions are predominantly from surface sources, which are described in Chapter 6 for CO₂, CH4, and N₂O. Direct use of observations of well-mixed GHG to model their regional budgets can also play an important role in verifying inventory estimates of emissions (Nisbet and Weiss, 2010). Systematic measurements of well-mixed GHG in ambient air began at various times during the last six decades, with earlier atmospheric histories being reconstructed from measurements of air stored in air archives and trapped in polar ice cores or in firn. In contrast to the physical meteorological parameters discussed elsewhere in this chapter, measurements of well-mixed GHG are reported relative to standards developed from fundamental SI base units (SI = International System of Units) as dry-air mole fractions, a unit that is conserved with changes in temperature and pressure (Box 2.1). This eliminates dilution by H₂O vapour, which can reach 4% of total atmospheric composition. Here, the following abbreviations are used: ppm = μmol mol–1; ppb = nmol/mol; and ppt = pmol/mol. Unless noted otherwise, averages of National Oceanic and Atmospheric Administration (NOAA) and Advanced Global Atmospheric Gases Experiment (AGAGE) annually averaged surface global mean mole fractions is described in Section 2.2.1 (see Supplementary Material 2.SM.2 for further species not listed here). Table 2.1 summarizes globally, annually averaged well-mixed GHG mole fractions from four independent measurement programs. Sampling strategies and techniques for estimating global means and their uncertainties vary among programs. Differences among measurement programs are relatively small and will not add significantly to uncertainty in RF. Time series of the well-mixed GHG are plotted in Figures 2.1 (CO₂), 2.2 (CH4), 2.3 (N₂O), and 2.4 (halogen-containing compounds).